The present invention is directed to a semiconductor optical resonator having a non-continuous waveguide and a phase shift element selectively movable into and out of an optical path defined through and by the resonator.
Optical resonators are of great interest in the telecommunication industry because of their ability to couple specific wavelengths from a multi-wavelength optical signal (e.g., WDM, DWDM, UDWDM, etc.) and route those wavelengths to a desired destination. A typical optical cross-connect includes input and output waveguides arranged so that a portion of each of the waveguides is disposed adjacent to a resonator. A desired wavelength of a multi-wavelength optical signal propagating in an input waveguide may be coupled to a particular output waveguide by providing a resonator tuned or tunable to that desired wavelength. Other wavelengths in the optical signal not on-resonance with a particular resonator will continue to propagate through and along the input waveguide and may ultimately be output therefrom. Consequently, a resonator can serve as a wavelength-specific routing device which guides particular wavelengths of light from an input path to one of several output paths.
The resonant wavelength for a circular resonator may be determined using equation (1):                     λ        =                              2            ⁢            π            ⁢                          xe2x80x83                        ⁢            Rn                    m                                    (        1        )            
In equation 1 R is the resonator""s radius, n is the effective index of refraction of the optical signal, and m is an integer of value 1 or greater.
If the resonator is not circular, the resonant wavelength is given by the equation (2):                     λ        =                  Ln          m                                    (        2        )            
In equation 2 L is the resonator""s length, n is the effective index of refraction of the optical signal, and m is an integer of value 1 or greater.
Today resonators are frequently employed as part of the cross-connect architecture of optical networks, such as depicted in FIG. 14. Resonators are especially well-suited for use in optical data and tele-communication systems such as, for example, DWDM systems. These systems efficiently transmit data by simultaneously transmitting a plurality of wavelengths of light over a single optical fiber or waveguide and then, selectively coupling a desired wavelength from the multi-wavelength signal at a desired location and routing that wavelength to a desired destination.
Cross-connect waveguide architecture is described in International Patent Appln. No. WO 00/50938, entitled xe2x80x9cVertically Coupled Optical Resonator Devices Over a Cross-Grid Waveguide Architecturexe2x80x9d.
For example, and as depicted in FIG. 14, a Mxc3x97N optical cross-connect 35 provides a plurality of input waveguides 37 (M1 and M2), a plurality of resonators 41, and a plurality of output waveguides 39 (N1, N2, and N3). Each resonator 41 may be tuned (or tunable) to a desired wavelength (i.e., an on-resonance wavelength) and thus couple only that wavelength from the multi-wavelength optical signal propagating in and through either input waveguide 37. That wavelength is then coupled from the resonator 41 to the corresponding output waveguide 39. Off-resonance wavelengths are not coupled and thus continue to propagate through the input waveguide 37.
Each of the input waveguides 37 may receive and transmit an optical signal from/to a long-distance transmission medium (e.g., a fiber-optic cable). Similarly, each of the output waveguides 39 may connect to a long-distance transmission medium. For example, each of the output waveguides 39 may connect to a fiber-optic cable over which an optical signal having a single wavelength may propagate.
Since the different wavelengths provided in the multi-wavelength optical signal are intended for different destinations, it is necessary to separate and suitably route each of those different wavelengths as separate and distinct optical signals. Resonators 41 perform this routing function quickly and efficientlyxe2x80x94since each resonator 41 can couple a particular wavelength of light traveling in an input waveguide 37 to an output waveguide 39.
If the resonators used in a cross-connect can only separate out a single wavelength of light, it will be necessary to provide the cross-connect with Mxc3x97N resonators. However, if the resonators can be tuned sufficiently, each of the resonators could be selectively tunable to a plurality of wavelengths, thereby eliminating the need for some resonators and simplifying the cross-connect structure.
It will be appreciated that the terms xe2x80x9cinputxe2x80x9d and xe2x80x9coutputxe2x80x9d are used for convenience, and that light could be transmitted in the opposite manner, that is, from the xe2x80x9coutputxe2x80x9d waveguide to the xe2x80x9cinputxe2x80x9d waveguide.
To be useful to the telecommunication market, resonators should meet two basic requirements: small size; and high tunability range.
Small size is desirable for two reasons. First, small resonators require less wafer real estate, which reduces costs. Second, small resonators have large free spectral range (FSR) characteristics, as is clear from equation 2:                     FSR        =                  λ          ⁢                      λ                          (                              2                ⁢                π                ⁢                                  xe2x80x83                                ⁢                Rn                            )                                                          (        3        )            
where xcex is the wavelength of the optical signal, R is the radius of the resonator and n is the effective refractive index of the medium through which the optical signal propagates (i.e., the resonator material). Referring to FIG. 16A, the FSR of a resonator having a 10 xcexcm radius is graphically depicted. Such a resonator has a FSR that accommodates approximately twenty-five 200 GHz channels in an optical signal transmitted at a wavelength of 1550 nm. In contrast, a resonator having a radius of 40 xcexcm has a FSR that accommodates approximately six 200 GHz channels, as depicted in FIG. 16B. A large FSR may be preferred because it allows for a higher number of optical channels to be multiplexed in a single fiber, resulting in better utilization of the fiber optical bandwidth.
Resonator operation can be enhanced if the resonator""s resonant wavelength can be varied, i.e., a tunable resonator, as that enables selective modification of the resonator""s switching behavior. Since the resonant wavelength of a resonator is related to the material from which the resonator is constructed, and to its index of refraction, changing the resonator index of refraction yields a corresponding change in the resonator resonant wavelength.
Increased resonator tunability is generally desirable because it provides a network administrator with the opportunity to reconfigure the network on the fly (without interrupting service) according to usage considerations and the demands of their clients. For example, and with reference to FIGS. 15A and 15B, any resonator in an optical cross-connect may be selectively tuned to a desired resonant wavelength. As depicted in FIG. 15A, the left-most resonator (in the figure) may be tuned to wavelength xcex2, and right-most resonator to wavelength xcex1. That configuration may be selectively changed, as desired, so that the left-most resonator (in the figure) may be tuned to wavelength xcex1, and the middle resonator to wavelength xcex2, as depicted in FIG. 15B.
Resonator tunability is important for another reasons. Using currently available semiconductor fabrication processes and techniques, resonators cannot easily be manufactured with the dimensional precision required to insure that the resonators perform as required. Resonator size is important because as explained below a resonator""s radius directly affects the resonator""s resonant wavelength, and, at least for telecommunication applications, resonator wavelength is strictly specified by the ITU grid, a telecommunication standard which specifies a plurality of optical channels that are typically separated by fractions of a nanometer. Dimensional variations caused by the resonator manufacturing process may cause resonators to have resonant wavelengths that do not meet the ITU grid requirements. For example, a 10 nm variation for a nominal 10 xcexcm radius resonator (and this presses the limits of what can be achieved using optical lithography), results in a resonant wavelength which deviates by 1.55 nm from the intended resonant wavelength. A deviation of this magnitude is not desirable, and in fact, may not even be acceptable in current telecommunication networks.
Such manufacturing variations might, however, be tolerable if the resonator could be tuned sufficiently to compensate for such manufacturing variations. Known tuning techniques, which are discussed in more detail above, do not permit sufficient tuning to compensate for all such manufacturing variations.
There are several ways to change a resonator""s index of refraction and so control the resonator""s operating wavelength, including thermally, by current injection, and by the electro-optic effect, each altering the resonator""s index of refraction and thus optical length. All of these techniques, however, have limitations.
For thermal tuning, the resonant wavelength shift may be determined using the following equation:                     Δλ        =                  λ          ⁢                                    Δ              ⁢                              xe2x80x83                            ⁢                              n                ⁢                Δ                            ⁢                              xe2x80x83                            ⁢              R                        nR                                              (        4        )            
where xcex94xcex represents the shift in resonant wavelength, xcex represents the resonator""s nominal resonant wavelength, n represents the resonator""s index of refraction, xcex94n represents the change in the resonator""s index of refraction, R represents the resonator""s nominal radius at a given temperature, and xcex94R is the change in the resonator""s radius induced by a temperature change.
In accordance with equation (3), the optical resonance wavelength is a function of both the resonator geometry and the waveguide refractive index. Consequently, there are several techniques by which a resonator may be tuned. By way of example, to change the resonance wavelength, either the index of refraction or the physical optical path length (given in Equation 1 as n and R, respectively) can be altered.
The index of refraction of the waveguide material can be altered by changing the waveguide""s temperature (thermal tuning), injecting current (current tuning) into the waveguide, or applying voltage to the waveguide (electro-optic tuning).
Thermal tuning is discussed in Rafizadeh, D., et al., xe2x80x9cTemperature Tuning of Microcavity Ring and Disk Resonators at 1.5-xcexcmxe2x80x9d, IEEE publication number 0-7803-3895-2/19 (1997).
In the case of either current injection or electro-optical tuning, the resulting change in resonance wavelength is:                     Δλ        =                  λ          ⁡                      (                                          Δ                ⁢                                  xe2x80x83                                ⁢                n                            n                        )                                              (        5        )            
Again, xcex is the wavelength of the optical signal, xcex94n is the change in the resonator material""s index of refraction, and n is the effective index of refraction of the resonator material.
A common semiconductor waveguide construction for implementing either current injection or electro-optic tuning involves doping the upper cladding with p-type dopant, the waveguide core with low or intrinsic dopant, and the lower cladding and substrate with n-type dopant. If electric contact is made to the upper (p-type) and lower (n-type) waveguide layers, the resulting p-i-n junction may then be operated in forward- or reverse-bias mode. Under forward bias, a change in the index of refraction of the waveguide core may be induced through current injection. Under reverse bias, a high electrical field can be formed across the intrinsic waveguide core and a refractive index change can result through the electro-optic effect. Both of these effects provide only a relatively small tuning effect.
Control over the resonator resonant wavelength using any of the above-described methods is limited, and may not provided sufficient tunability to account for possible manufacturing variations affecting the resonator size (i.e., radius).
It will be understood that tuning using these methods generally yields tuning ranges below 1 mn.
Consequently, while tuned and tunable resonators are generally known, there exists a need in the art for a tuned and/or tunable resonator that is relatively small in size and which can be tuned across a wider range of wavelengths. More particularly, there is a need for a resonator that can be tuned such that the resonator""s operating wavelength varies by at least approximately an order of magnitude more than the 1 nm tuning range currently achievable.
The present invention is directed to a method of tuning a resonator and to a tunable resonator having a non-continuous waveguide having first and second confrontingly opposite end faces between which is defined a region. A continuous optical path is defined through the waveguide and across the region. A phase shifter is provided that is selectively movable in the region into and out of the optical path. When located in the optical path, the phase shifter introduces a phase shift in an optical signal propagating through the waveguide and along the optical path.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.